> using function syntax for methods

hmm, yeah fair enough I suppose. I don't think I've found a good use-case for that yet. I guess having the symmetry there makes the feature easier to explain at least? I dunno.

> Furthermore, I don’t think it necessarily makes sense for all functions that happen to take, say, a string as their first argument, to be listed in the code completion for method invocation on a string variable.

All functions in scope that happen to take a string as their first argument. If this turns into an actual problem in practice it's quite doable to refactor things such that it's not an issue.

I find that when I use autocomplete I'll be typing the first bit of the method name in any case. I never "browse" autocomplete to look for the thing I need.

Extension methods are another way to do the same thing yes, but that feels like special-casing behavior, where UFCS is more general. With extension methods you need to think of how to implement it wrt things that don't usually have methods attached. With UFCS that just works the way you'd expect it to.